On conventional roller dynamometers it is possible merely to measure the net power of the vehicle engine acting on the vehicle wheels, but this net power includes the internal power loss of the respective vehicle. Basically, the amount of such power loss could be calculated on the basis of the resistances and the moments of inertia effective in the input train so that the actual gross power of the vehicle engine could be determined by addition of the measured net power and the calculated power loss. However, this method is not feasible in practice because the power loss depends on a large number of different parameters which vary for different vehicle types and also depends on the condition of the individual vehicle, such as wear of the components rotating in the input train, viscosity of the lubricants employed and condition of the bearings, among others.
On the other hand, the exact determination of the gross power of the vehicle engine provides information as to the respective state of the engine when the established values are compared with the manufacturer's specifications. The same holds in respect of a practical determination of the power loss which permits inferences on the state of the power transmission, for example, the manual transmission, the differential, the bearings, etc. Until recently, the gross power of a vehicle engine could be determined with sufficient accuracy only after decoupling of the input train and by means of power measurement direct at the engine output shaft. But such a test can only be performed on special engine test stands, and normally it can only be performed with a completely dismounted drive assembly. The effort required therefor usually is beyond the means of normal repair and maintenance shops.